.
Angewandte
Communications
Pentacenes
Stabilizing Pentacene By Cyclopentannulation
Sambasiva R. Bheemireddy, Pamela C. Ubaldo, Peter W. Rose, Aaron D. Finke,
Junpeng Zhuang, Lichang Wang, and Kyle N. Plunkett*
Abstract: A new class of stabilized pentacene derivatives with
externally fused five-membered rings are prepared by means of
a key palladium-catalyzed cyclopentannulation step. The target
compounds are synthesized by chemical manipulation of
a partially saturated 6,13-dibromopentacene precursor that
can be fully aromatized in a final step through a DDQ-
mediated dehydrogenation reaction (DDQ = 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone). The new 1,2,8,9-tetraaryldicyclo-
penta[fg,qr]pentacene derivatives have narrow energy gaps of
circa 1.2 eVand behave as strong electron acceptors with lowest
unoccupied molecular orbital energies between À3.81 and
À3.90 eV. Photodegradation studies reveal the new compounds
are more photostable than 6,13-bis(triisopropylsilylethynyl)-
pentacene (TIPS-pentacene).
Figure 1. Pentacene structures. According to Clar’s sextet rule, 1, 3,
H
igher acenes are known to readily undergo photoinduced
and 4 have a single migrating p sextet while 2 has four p sextets.
oxidation, dimerization, or polymerization largely owing to
a biradical character in the electronic ground state.[1–3] These
decomposition pathways are undesirable because acene
stability has been identified as a critical design parameter
for their incorporation in electronic devices,[4] considering
that acene-based devices have been found to fail as a result of
decomposition of the active material.[5] To address this
problem, useful strategies to stabilize acenes have employed
ethynylation,[6–9] thiolation,[10–12] cyanation,[13,14] or bulky sub-
stituent incorporation[15–18] to access electronically modulated
or sterically blocked structures.[19] Benzannulation, or higher
ordered six-membered ring fusions, of the acene core is an
alternative strategy to obtain stabilized compounds, yet the
resulting electronic properties of these derivatives no longer
possess the desirable acene character. For example, upon
of the polycyclic aromatic hydrocarbon (PAH).[21] Although
benzannulated 2 contains four isolated p sextets, a pentannu-
lated acene core (for example as in 3) alleviates this
conjugation dilemma owing to the isolation of a double
bond in the five-membered ring. From the Clar sextet
representation, the electronic structure of 3 should be similar
to that of a native pentacene with a single migrating p sextet
(Figure 1). Owing to our recent interest in cyclopenta-fused
polycyclic aromatic hydrocarbons (CP-PAHs),[22–26] we set out
to prepare pentacene derivatives working under the hypoth-
esis that the fusion of five-membered rings would minimize
the biradical character at the central carbons leading to
stabilized structures. Towards this new class of compounds, we
demonstrate herein that cyclopentannulation of a partially
unsaturated pentacene precursor followed by an aromatiza-
tion step with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) leads to the formation of new pentacenes that are
more stabilized than the well-known 6,13-bis(triisopropyl-
silylethynyl)pentacene (4; TIPS-pentacene). These molecules
behave as strong electron acceptors and possess significantly
decreased energy gaps.
benzannulation of pentacene
1
to form dibenzo-
[fg,qr]pentacene 2,[20] significant electronic modulation
occurs and the molecule no longer behaves as an acene but
rather as two electronically separated phenanthrene units
(Figure 1). These properties can be easily rationalized by
invoking Clarꢀs aromatic sextet rule that says the resonance
contributor with the most p sextets (that is, benzene-like
moieties) is the most important contributor to the properties
The palladium-catalyzed cyclopentannulation chemistry
utilized here is an extension of our recent work[26] that was
rooted in the contributions of several other groups.[27–31] For
this work, we needed to adapt the pentannulation synthetic
method from an anthracene to a pentacene core. This new
chemistry required the synthesis of 6,13-dibromopentacene,
or a reactive surrogate, in place of the previously utilized 9,10-
dibromoanthracene. Owing to the unknown nature of 6,13-
dibromopentacene,[32] we turned to a pentacene precursor
that could undergo known transformations to access the
desired CP-PAH substructure and then could later be trans-
formed into the desired pentacene derivatives. To accomplish
[*] S. R. Bheemireddy, Dr. P. C. Ubaldo, P. W. Rose, Prof. J. Zhuang,
Prof. L. Wang, Prof. K. N. Plunkett
Department of Chemistry and Biochemistry and the Materials
Technology Center, Southern Illinois University
Carbondale, IL 62901 (USA)
E-mail: kplunkett@chem.siu.edu
Dr. A. D. Finke
Swiss Light Source, Paul Scherrer Institute
Villigen PSI (Switzerland)
Supporting information for this article is available on the WWW
15762
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15762 –15766